MOLTEN SALT REACTORS - SAFETY OPTIONS GALORE
Uri Gat
Oak Ridge National Laboratory
H. L. Dodds
University of Tennessee at Knoxville
ABSTRACT
Safety
features and attributes of molten-salt reactors (MSR) are described.
The unique features of fluid-fuel reactors of on-line continuous
processing and the ability for so-called external cooling result in
simple and safe designs with low excess reactivity, low fission-product
inventory, and small source term. These, in turn, make a criticality
accident unlikely and reduce the severity of a loss-of-coolant accident
to where they are no longer severe accidents. A melt-down is not an
accident for a reactor that uses molten fuel. The molten salts are
stable, non-reactive and efficient heat-transfer media that operate at
high temperatures at low pressures and are highly compatible with
selected structural materials. All these features reduce the accident
plethora. Freeze valves can be used for added safety. An ultimate-safe
reactor (USR) is described with safety features that are passive,
inherent and non-tamperable (PINT).
I. INTRODUCTION
The
Molten-Salt Reactors (MSR) that are the subject of this paper, are
fluid-fuel reactors (FFR) that utilize primarily fluoride salts as
their working fluid. These reactors have the fissile material, as a
salt, homogeneously mixed in the carrier salt. Fluid-fuel reactors
differ fundamentally from solid-fuel reactors. Some of the more
important differences are that the fuel can be readily processed on
line to
remove, or add, selective components. This processing
differs from solid-fuel reprocessing where the entire fuel (elements)
must be removed, treated, remanufactured into elements, and reinserted
in the reactor. In contrast, the processing of fluid fuel can consist
of continuous removal of gases and volatiles in an on-site processing
of a selected side-stream. Another important difference is the fact
that the fuel itself can be the coolant and circulated to a heat
exchanger that
is external to the core. This cooling method is
referred to as external cooling. External cooling and on-line
processing are contributors to unique safety features of FFRs.
II. FLUID FUEL REACTORS
It
was recognized from their inception that FFRs possess unique and
desirable safety features. Some of these features are: Simple structure
- this is particularly applicable for external cooling. The core can be
optimized for nuclear and safety, and there is no need for compromise
to accommodate coolant and heat exchanger surfaces. FFRs can have
continuous removal of fission products. This feature dispenses with the
need for excess reactivity to compensate for burnup and poisoning,
removing the source of reactivity excursions, and reducing the source
term and driving force (after-heat) for an accident. They also possess,
"Inherent safety and ease of control." The inherent safety refers to
the high negative reactivity temperature coefficient associated with
the expansion of the fluid upon heating and resulting in the expelling
of fuel from the core to reduce the reactivity. This response is
limited by the speed of sound propagation (shockwave). Combined with
low excess reactivity the FFRs can be self-controlling. They can
operate with no externally operated controls, thus the safety can be
passive, inherent and non-tamperable (PINT). Control rods may be used
in FFRs to control the operation temperature. Ultimate shut-down is
accomplished by draining the fuel, by gravity, from the critical
configuration in the core to guaranteed sub-critical configurations in
drain tanks. These features have been demonstrated in the operation of
the Molten-Salt Reactor Experiment (MSRE).
There are safety
concerns associated with FFR: "Possible fluctuations of reactivity
caused by density or concentration changes in the fuel, e.g.,
bubbling." For MSRs this concern is primarily the coalescence of
dissolved gas into large bubbles and their collapse, or in some
concepts, such as the MSBR, the expansion of bubbles. To assure that
this does not occur, continuous removal of gaseous (fission products)
must be employed, usually through sparging. Early concerns of loss of
delayed neutrons, which are carried out of the core in external
cooling, turned out to be of no significance.
III. MOLTEN SALT REACTORS
The
molten salts considered for MSRs are chemically stable. They do not
react rapidly with moisture or air. Their chemical inertness precludes
accidents that are due to chemical interaction. There is no fire hazard
or explosion hazard. They are also compatible and are non-corrosive
with respect to suitable structural materials. The experience with the
MSRE has shown that high-nickel alloys, combined with adequate
oxidation potential balancing of the salt, can result in low corrosion
of the structural materials.
The molten salts considered for the
MSR are stable to high temperatures at low pressures. This feature
allows for high efficiency with no extreme safety demands from the
structure materials. Being a liquid system at low-pressure eliminates
the storage of potential energy or other risk of an energetic burst or
explosion. Molten salts are often used in industry as heat transfer
media for their inertness and safety. There is ample experience in
handling molten salts.
Small spills are not a source of a major
accident as there are no violent reactions that can accompany a spill.
As a spill occurs, the salt is spread out and cools more efficiently
than in the insulated pipes. The salt freezes in place without
spreading and is available for recovery operation. The freezing process
is inherent and passive. Should there be some residual heat sources in
the salt, it will stay molten until it reaches a configuration in which
the thermodynamic equilibrium brings it to a freeze.
IV. FREEZE VALVES
The
MSR can utilize freeze valves in critical locations or where desired.
Freeze valves can be ordinary sections of pipe which are exposed to a
cooling stream of environmental gas to the extent that it creates a
frozen plug that blocks the flow and acts as a valve. Where such a
valve has a safety function, as in draining the fuel to the storage
tanks, it is prudent to design it such that the required flow is
gravity-driven.
The frozen valve itself can be designed such that when the salt rises
above a certain predetermined temperature the heat overrides the
cooling, melts the frozen plug and opens the valve. Such an arrangement
is passive, inherent and non-tamperable (PINT-safe).
Furthermore,
the properly sized external cooling of the freeze valve cooling drive,
such as an electric driven fan, will cease with any failure of the
power and release the valve to melt and perform its safety function.
This mode of operation is again PINT-safe.
V. SEVERE ACCIDENTS
For
nuclear reactors it is common to consider three types of severe
accidents: criticality accident, failure to remove after-heat and a
meltdown. The meltdown is not an accident by itself but rather a
description of a consequence of an accident. The concern with a
meltdown is the possibility of breach of containment and release of the
source term, and also a rearrangement of the fuel into a re-critical
configuration. For the MSR the fuel melting is, of course, a moot issue
since the fuel is in a molten state in its normal operating
configuration. A possible advantage of the MSR is that the fuel is
subject to freezing, upon breach of a vessel or pipe, and its
dispersement. The fuel will disperse, and thus increase its cooling
geometry, until it reaches a freezing configuration and thus will be
confined to that location and configuration. The design of the MSR must
account for such a situation so that recovery, by collecting the fuel
and correcting the failure that led to the dispersal, is simple and
readily possible. The issues of the source term, re-criticality, and
after-heat removal are discussed in the respective following paragraphs.
VI. THE SOURCE TERM
The
source term, which is the inventory of radioisotopes in the reactor
available for dispersion to the environment, contributes two-fold to an
accident. The source term is the measure of the radiation which needs
to be contained from reaching any sensitive location or target. The
energy contained in the source term also provides the driving force for
the dispersion of the source term as it is also a measure of the after
heat, or the energy, to damage a reactor in the event of heat-removal
failure or loss-of-coolant accident (LOCA). For an MSR, as for any
fluid fuel reactor, on-line fuel processing can be applied. The on-line
processing, at the least, removes the gaseous and volatile part of the
source term. This part is the most likely to be dispersed when there is
a breach of containment. Fuel processing also reduces the inventory of
longer and long-lived isotopes as their accumulation is time dependent.
The MSRs processing can be adjusted to have a small source term. The
safety advantages of this small source term are many fold: The driving
force for dispersion is reduced; the gaseous and volatile components,
which are the most likely to disperse, are essentially all but
eliminated; the long half-life isotopes (elements) are reduced such
that the long-term effect of even the most unlikely accident is not
severe; and, the short-lived isotopes require a proportionately
short-term protection time till they decay. Thus, even a hypothetical
severe accident is ameliorated a priori.
A properly designed
processing facility quickly removes the separated radioisotopes from
the purview of the reactor. This makes them totally unavailable to the
reactor source term even under the most extreme hypothesized
circumstances.
VII. CRITICALITY ACCIDENT
In MSRs
with processing, the criticality accident is essentially eliminated
(See concerns section for exceptions.). There are two factors that make
an excess reactivity incident unlikely, temperature control and
optimized geometry. The MSR can be temperature-controlled. The large
negative temperature coefficient allows for control without control
rods or other mechanically operated control mechanism. The operability
of the reactor under temperature control has been demonstrated on
FFR(HRT). The control rods can be used for temperature regulation.
Continuous fuel
processing, with the ability to externally add
fissile material when needed, reduces the need for excess reactivity
inventory. There is no need to compensate for burnup as the poisoning
fission products are kept at (low) equilibrium. The simple design,
particularly when utilizing external cooling, eliminates the
possibility of shifting or rearranging materials to result in an
increased reactivity. The absence of coolant per se does not provide
room that could be filled with shifting fuel to increase reactivity.
The MSR can be designed so that bred fuel, at a breeding ratio of 1.0,
keeps the reactor at equilibrium with fertile-material feed and with no
need to add fissile material. Since the fuel is also the coolant, the
reactor is largely temperature-controlled regardless of the power.
The
adequately-designed MSR has an optimum geometrical design for
criticality in the core. The externally-cooled reactor has neither
coolant nor structural materials in the core that may require design
compromises and thus can truly be optimized for safety. This core
optimization also assures that no criticality, or re-criticality,
outside the core can occur.
VIII. AFTER HEAT ACCIDENT
The
MSR can be designed, with sufficiently rapid processing, that it can
contain adiabatically the entire inventory after-heat without reaching
boiling. Furthermore, since the fuel is the coolant, in external
cooling, a LOCA has no meaning. As a rule, natural convection cooling
could be designed but may not be desirable as the
temperature-controlled reactor will maintain its design temperature
regardless of the power. The reaction needed is to drain the fuel, by
gravity, into dump tanks
that are assured to retain subcriticality
and have sufficient natural cooling to assure cooling of the fuel. The
activation of the draining can be done by means of freeze valves that
assure PINT safety for after heat removal.
IX. CONCERNS
There
are two safety concerns for the MSR that can lead to a power excursion.
The first of these concerns is the accumulation of gases and volatile
materials in the fluid fuel that would coalesce into bubbles that could
then collapse at once in the core, resulting in a reactivity excursion.
A careful design will ensure that such an event is avoided. The
dispersed gases must accumulate over an extended period of time, which
allows for removal by sparging, and recognizing and noticing the
failure of the gas and volatile removal system. By removing the gases
early in the cycle of the fuel from the core to the heat exchanger, the
likelihood of the collapse of a bubble in the core can be minimized.
The geometrical design of the core can also assure that the added
volume has a small reactivity contribution.
The second concern
is the cold slug accident. A core with little or no fuel circulation
will remain at criticality, while the external fuel can cool down to
low temperatures. A sudden reestablishment of the circulation will
introduce a slug of cold fuel to the core. Due to the large negative
temperature coefficient, this cold fuel represents a reactivity
excursion that will result in a power burst. The primary pump, or
absence thereof, must be carefully sized to assure that such an
excursion does not exceed the design margins of the reactor.
X. THE ULTIMATE SAFE REACTOR (USR)
The
Ultimate Safe Reactor (USR) is a special concept of a molten-salt
reactor with prime and complete emphasis on safety. The USR uses a
processing frequency, yet to be developed, that is about an order of
magnitude higher from that contemplated for the molten salt breeder
reactor (MSBR). The MSBR had a ten-day inventory turn around in the
fuel processing. The USR uses a one day or less of turnaround of the
fuel
inventory. This rather fast turnaround reduces the build up of
all fission products with half-lives of a few days or longer. The
reactor is an epithermal spectrum reactor and uses no moderator per se
in the core. The clean core consists solely of a low-pressure vessel.
Freeze valves are used throughout. The prime circulating pump is sized
to assure no critical cold slug accident can occur. Furthermore, the
USR uses the Th-U fuel cycle with a breeding ratio of exactly one.
Thus, the USR has all the safety benefits that are passive, inherent
and non-tamperable and, in addition, has proliferation-resistant
attributes and simplified waste that is free of fissile material, which
can be transported in any arbitrary size or quantity from the
processing part of the plant.
The USR has no control rods and is
temperature controlled by elevation of fuel in the core. The start-up
procedure is the pumping of the fuel from its storage or dump tanks
into the core. The small pump that accomplishes this transfer is sized
such that at maximum capacity the temperature rise rate of the core is
within the design limits.
XI. THE ABSOLUTE AND ULTIMATE SAFE REACTOR (A+USR)
The
absolute and ultimate safe reactor (A+USR) is a special concept of the
USR which utilizes natural convection to transfer the heat from the
core to the heat exchanger. The A+USR has no safety-related mechanical
operating parts nor any externally-actuated controls, it becomes the
ultimate in PINT-safety. The reactor responds internally and inherently
to a change in power demand via its temperature response.
Frequent
processing of the fuel increases the fuel inventory in the processing
part and puts high demand on the performance of the processing units.
The removal of the fission products from the fuel stream occurs at low
concentrations, which requires precision and sophistication. In an
actual plant, an optimization between performance, inventory and safety
is needed.
XII. SUMMARY
The molten-salt reactor
with fuel processing can be designed to be almost as safe as desirable.
The basic features of fluoride based molten salts allow for a high
temperature, and thus efficient, operation at low pressures. The molten
salts are inert and well compatible with selected structural materials.
The MSR is not subject to safety concerns from chemical or mechanical
violent reactions or explosions. External cooling results in a simple
design with few structural requirements that permits optimization of
the design for safety-eliminating compromises. The on-line processing
results in an equilibrium fuel that requires no excess reactivity for
burn-up or poison compensation. The fission product inventory, and
therefore the source term, is held low. The severe accidents of
uncontrolled super-criticality or loss-of-cooling that fails to remove
the after-heat can become a hypothetical accident.
The dreaded
meltdown looses all its meaning in a fluid-fuel reactor. In an MSR, a
spill may be self-containing by the freezing of the fuel upon cooling.
Freeze valves are one more feature that can make an MSR PINT (passive,
inherent, non-tamperable) safe.
The USR and the A+USR are
concepts that bring together the safety features of an MSR and result
in a reactor with safety features that are beyond current requirements
and expectations.
ACKNOWLEDGMENT
The authors wish
to express their thanks and gratitude to J. R. Engel, who has reviewed
this paper and provided extremely valuable comments and corrections.
Many of the ideas expressed in this paper were developed by Dick and
were explained to the authors over the course of many years.
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